Is it possible to achieve Buffalo Plaid Texture with Checker Node?
This one will allow you to change size and any of the three colors with only one node.





EDIT:

I've improved this method so it's now possible to group it and use it as just one node with all the controls:



And this is what is grouped:



What is basically doing is generating black-gray-white pattern, assign RGB values to these colors by Color Ramp than separating them and allow to change them.

Blend file:



First Answer:

I don't know if this is the simplest way, but you can make something like this using two Wave Textures.



Value node controls checker size and Color Ramp is used to change colors.